Cycling of matter and energy in ecosystems

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Energy Flow in Ecosystems

Energy flows through an ecosystem in a **one-way** direction, from the sun to producers to consumers.
**Producers** (plants, algae) capture sunlight energy and convert it into chemical energy (glucose) via **photosynthesis**.
**Consumers** obtain energy by eating producers or other consumers; they are classified as primary, secondary, or tertiary consumers.
**Decomposers** (bacteria, fungi) break down dead organisms and waste, releasing nutrients back into the environment.
Energy is **lost as heat** at each trophic level due to cellular respiration and other metabolic processes.
Only about **10%** of the energy at one trophic level is transferred to the next; the rest is used for life processes or lost as heat.

Trophic Levels and Food Chains

A **food chain** shows the linear flow of energy from one organism to another (e.g., grass → rabbit → fox).
Each step in a food chain is a **trophic level**: producers, primary consumers, secondary consumers, tertiary consumers.
Arrows in a food chain point in the direction of **energy flow** (from the eaten to the eater).
Most ecosystems have **complex food webs** with many interconnected food chains.
**Trophic pyramids** (energy pyramids) illustrate the decreasing amount of energy available at each higher level.

Photosynthesis and Cellular Respiration

**Photosynthesis**: carbon dioxide + water + sunlight → glucose + oxygen. Occurs in chloroplasts of producers.
**Cellular respiration**: glucose + oxygen → carbon dioxide + water + energy (ATP). Occurs in all living cells.
These two processes are **opposite reactions** and form a cycle: the products of one are the reactants of the other.
Photosynthesis stores energy; cellular respiration releases it for use by organisms.

Matter Cycling: Carbon and Nutrients

Matter (atoms) is **recycled** within ecosystems; it is not created or destroyed, only transformed.
The **carbon cycle**: carbon moves between the atmosphere, organisms, and the Earth through photosynthesis, respiration, decomposition, and combustion.
Decomposers break down dead matter, returning carbon and other nutrients (like nitrogen and phosphorus) to the soil or atmosphere.
Plants absorb nutrients from the soil; these nutrients are passed through the food web and eventually returned by decomposers.

Primary Production and Efficiency

**Gross primary productivity (GPP)** is the total energy captured by producers through photosynthesis.
About **60%** of GPP is used by producers for respiration; the remainder is **net primary productivity (NPP)**.
NPP is the energy available to consumers (stored as biomass in producers).
**Ecological efficiency** (energy transfer between trophic levels) typically ranges from **5% to 20%**.
Only **1%** of the sun's energy that reaches Earth is captured by producers.

Chemosynthesis in Extreme Environments

**Chemosynthesis** is a process where bacteria use chemical energy (e.g., from hydrogen sulfide) instead of sunlight to produce glucose.
This occurs in deep-sea hydrothermal vents and other dark environments.
Chemosynthetic bacteria serve as **producers** in these ecosystems, supporting unique food webs.

Carnivorous Plants: Adaptations for Nutrients

Carnivorous plants (e.g., Venus flytrap, pitcher plant) live in nutrient-poor soils and obtain **nitrogen** by trapping and digesting insects.
They still perform **photosynthesis** for energy but supplement their nutrient intake.
Trapping methods include pitfall traps, flypaper traps, and snap traps.

Energy pyramid showing the 10% energy transfer between trophic levels.

The cycle of photosynthesis and cellular respiration showing the flow of energy and matter.

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Practice questions

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1.What is the original source of energy for most ecosystems?
Easy
- Athe sun
- Bthe soil
- Cwater
- Dcarbon dioxide
2.Which organisms are classified as producers in an ecosystem?
Easy
- Aorganisms that perform photosynthesis
- Borganisms that eat other organisms
- Corganisms that break down dead matter
- Dorganisms that only respire
3.What happens to energy as it moves from one trophic level to the next?
Medium
- AIt increases because of heat absorption.
- BIt remains constant.
- CMost energy is lost as heat and some is used for respiration.
- DIt is completely transferred to the next level.
4.In a food chain, what do the arrows represent?
Medium
- Athe direction of energy flow
- Bthe size of organisms
- Cthe number of organisms
- Dthe direction of matter cycling
5.In an ecosystem, if the gross primary productivity (GPP) of a producer is 1000 kcal/m²/year and the net primary productivity (NPP) is 400 kcal/m²/year, how much energy is used for cellular respiration by the producer?
Hard
- A400 kcal/m²/year
- B600 kcal/m²/year
- C1000 kcal/m²/year
- D1400 kcal/m²/year
6.Chemosynthetic bacteria obtain energy from:
Medium
- Asunlight
- Bchemicals like hydrogen sulfide
- Cdead organic matter
- Dcarbon dioxide
7.Which of the following is a consumer?
Easy
- Agrass
- Balgae
- Crabbit
- Dtree
8.What is the approximate percentage of energy transferred from one trophic level to the next?
Medium
- A50%
- B10%
- C90%
- D1%

Revision notes

## Cycling of Matter and Energy in Ecosystems - **Energy flow** is unidirectional: sunlight → producers → consumers → decomposers. Energy is lost as heat at each step. - **Trophic levels**: producers (autotrophs), primary consumers (herbivores), secondary consumers, tertiary consumers, decomposers. - **Food chains** show one path; **food webs** show interconnected paths. - **10% Rule**: Only about 10% of energy (5–20% efficiency) passes from one trophic level to the next. The rest is used for respiration or lost as heat. - **Photosynthesis**: \(6CO_2 + 6H_2O + \text{sunlight} \rightarrow C_6H_{12}O_6 + 6O_2\) - **Cellular respiration**: \(C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + \text{energy}\) - **Carbon cycle**: Carbon moves between organisms and the environment via photosynthesis and respiration. Decomposers return carbon to the soil/atmosphere. - **Primary production**: Gross primary productivity (GPP) = total energy captured by producers. Net primary productivity (NPP) = GPP – energy used by producers for respiration. NPP is available to consumers. - **Limiting factors**: Sunlight, water, nutrients (N, P) affect primary production. - **Chemosynthesis**: Some bacteria use chemical energy (e.g., hydrogen sulfide) to produce glucose, supporting deep-sea vent ecosystems. - **Carnivorous plants**: Obtain nutrients (e.g., nitrogen) by trapping and digesting insects, adapting to low-nutrient soils. **Key idea**: Matter cycles (e.g., carbon, water, nitrogen) while energy flows one way and is eventually dissipated as heat.

Trophic level: A step in a food chain or food web; each level represents organisms that share the same function in the energy flow (e.g., producers, primary consumers).
Food web: A network of interconnected food chains showing the flow of energy and matter among organisms in an ecosystem.
Photosynthesis: Process by which producers use sunlight, water, and carbon dioxide to produce glucose and oxygen: 6CO₂ + 6H₂O + sunlight → C₆H₁₂O₆ + 6O₂.
Cellular respiration: Process by which organisms break down glucose using oxygen to release energy, producing carbon dioxide and water: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O + energy.
Gross primary productivity (GPP): Total amount of energy captured by producers through photosynthesis.
Net primary productivity (NPP): Energy remaining after producers use some for respiration; NPP = GPP – respiration by producers.
10% rule: Only about 10% of energy at one trophic level is transferred to the next; the rest is lost as heat or used for respiration.
Chemosynthesis: Process by which certain bacteria use chemical energy (e.g., from hydrogen sulfide) to produce glucose, without sunlight.
Decomposer: Organism (e.g., bacteria, fungi) that breaks down dead matter, releasing nutrients back into the ecosystem.
Carnivorous plant: A plant that traps and digests insects or other small organisms to obtain nutrients, especially in low-nutrient environments.

Flashcards

What is the ultimate source of energy for most ecosystems?

The sun.

What is a producer?

An organism that performs photosynthesis, converting sunlight into chemical energy (glucose). Examples: algae, grasses, trees.

What is a consumer?

An organism that eats other organisms to obtain energy. Consumers cannot make their own food.

What is a food chain?

A sequence showing the transfer of energy from one organism to another, starting with a producer and moving through consumers.

What is a trophic level?

Each step in a food chain or food web, representing a feeding position (e.g., producer, primary consumer, secondary consumer).

What does the direction of an arrow in a food chain represent?

The direction of energy flow, from the organism being eaten to the one that eats it.

Is energy flow in an ecosystem one-way or cyclical?

One-way (unidirectional). Energy enters as sunlight, passes through organisms, and is lost as heat at each step.

What is the 10% rule in energy transfer?

Only about 10% of the energy at one trophic level is passed to the next; the rest is used for respiration or lost as heat.

Why are there usually fewer tertiary consumers than producers?

Because energy is lost at each trophic level, so less energy is available to support higher levels.

What is photosynthesis?

The process by which producers use sunlight, water, and carbon dioxide to make glucose and oxygen.

What is cellular respiration?

The process by which organisms break down glucose using oxygen to release energy, producing carbon dioxide and water.

How does the carbon cycle connect photosynthesis and cellular respiration?

Photosynthesis removes CO2 from the air and fixes carbon into glucose; respiration releases CO2 back into the air, allowing it to be reused by plants.

What is gross primary productivity (GPP)?

The total amount of energy captured by producers through photosynthesis.

What is net primary productivity (NPP)?

The energy remaining after producers use some for respiration: NPP = GPP – respiration.

What percentage of solar energy typically enters producers?

About 1%.

What is a trophic pyramid?

A graphical representation showing the relative amount of energy or biomass at each trophic level, with producers at the base.

What is chemosynthesis?

A process where bacteria use energy from chemicals (e.g., hydrogen sulfide) to produce glucose, often occurring at deep-sea hydrothermal vents.

How do chemosynthetic bacteria obtain energy?

They use chemical energy from compounds like hydrogen sulfide, rather than sunlight, to convert CO2 into glucose.

What are decomposers and their role in matter cycling?

Decomposers break down dead organisms and waste, returning nutrients (like carbon and nitrogen) to the soil or air for reuse by producers.

Why is matter cycled but energy flows one way?

Matter (atoms) is recycled through ecosystems via processes like decomposition and photosynthesis, while energy is transformed and eventually lost as heat, requiring a constant input.

What are two nutrients that can limit primary production?

Nitrogen (N) and phosphorus (P).

What is a carnivorous plant?

A plant that traps and digests insects to obtain nutrients like nitrogen, allowing it to survive in nutrient-poor environments.

Name three types of traps used by carnivorous plants.

Pitfall trap (pitcher plant), flypaper trap (sundew), and snap trap (Venus flytrap).

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